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Dracula Technologies finalizes an additional fundraising of 550,000 €.

Dracula Technologies finalizes an additional fundraising of 550,000 €.

Dracula Technologies has just raised nearly 550,000 euros on the WiSEED financing platform, in addition to the 1.9 million raised from the manufacturers MGI Digital Technology and Isra Cards.

Despite a complicated context and a financing period marked by confinement, this campaign was a real success.

Many testimonials of encouragement have marked us and encourage us to continue our efforts to make LAYER® the reference energy harvesting technology for powering indoor connected objects.

  • “I’m already a shareholder and I’m suing.”

    Albert D.

  • “I invested because I believe in the technology developed by Dracula and its market positioning. I would also like to salute the team who took the time to respond to each of the comments and questions on the forum with enthusiasm and pedagogy. How could a project led by such people not succeed? There seem to be many opportunities (IoT, cards, sensors, intelligent objects, advertising objects, signage, IT accessories…).”

    Marc A.

  • “The market for connected objects should explode, and Dracula Technologies’ LAYER technology should provide an interesting (economic and ecological) response to the problem of their energy supply”.

    Thibault L.

  • “Hello, I have been a shareholder since 2017 and I believe in this new technology. A reliable company with ambition. »

    Gregory A.

This additional financing will enable us to accelerate the pre-industrialisation of our LAYER® technology and to finance recruitment for key positions.

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Thin Film Processing Method: from lab to Industrialization of OPV Devices (2nd part)

Thin Film Processing Method: from lab to Industrialization of OPV Devices (2nd Part)

Second part: Printing techniques for industrialization

In the first part, we described the different techniques used widely in laboratories. In the second part, we go through the different techniques able to industrialize the OPV module production.

Gravure Printing

Gravure printing (also known as rotogravure printing) is used to print high-quality and high-volume printed matter and packaging. Like engraving, gravure is a form of intaglio printing that produces fine, detailed images. The gravure printing unit consists of five parts: gravure cylinder, impression cylinder, doctor blade system, ink fountain, and dryers.

Briefly, the pattern to be printed is engraved as a discrete cavity into a rotary printing cylinder. During the printing process, the engraved cavities are filled with the ink by passing an ink bath and a flexible doctor blade is used to remove the excess ink. A chambered doctor blade system can be used for inks containing highly volatile solvent. The ink on the printing roll is then deposited when the cylinder is brought into contact with the substrate. The film is passed through a dryer to drive off the solvent or water. The thickness of the printed layer is determined by the gravure cell volume and the pick-out ratio, leading to feature lines ranging from 0.2 to several micrometers. The advantages of the gravure printing technique are its inherently simple working principle, simple printing equipment, high production speed, stability of the process, high through-put, and high resolution. Additionally, various solvents can be used in gravure printing as the printing cylinders are made of chrome plated copper. Disadvantages of gravure printing are the high cost of cylinders, high requirements for suitable process parameters and high-quality demands of substrates (e.g. very smooth surface). Like flexography, gravure printing predominates in the high-volume printing of packaging, wallpaper and gift wrap. Although less common, it also works for printing magazines, greeting cards, and high-volume advertising pieces. Importantly, gravure printing has been already successfully used to produce laboratory- scale-processed OPV cells and modules 17, 18.

gravure-printing

Flexographic printing

Flexographic printing is a R2R technology. It uses several cylinders to depose a patterned OSCs layer to a substrate. This technique differs from gravure printing mainly in the fact that the transfer of the ink is performed from a relief opposed to cavities. The final pattern stands out from the printing plate which is typically made from rubber or a photopolymer. The flexo  system  consists  of  fountain  rollers  that  continuously  transfer  ink  to  the  ceramic  anilox  roller  which  has  engraved  cells/micro  cavities  embedded  into  the  exterior.  This allows the collection of ink which is then transferred to the relief on the printing cylinder that performs the final transfer to the web. The ink pick out from the anilox corresponding to the negative pattern of the motif.

This technique has been used for the printing of electrically conductive structures. Typically, inks for flexographic printing have a relatively low viscosity between 50 mPas and 500 mPas. The printing pressure is usually very low (“kiss printing”), making flexography an applicable technology for printing on rough-textured and breakable surfaces.

Despite not being studied as much in OPV research compared to other deposition methods, flexographic printing has seen some success19 and is advantageous due to easier patterning and faster coating when compared to other techniques like slot-die coating.

flexographic-printing

Screen-printing

Screen-printing is a popular technique used in a whole range of different industries, so even if you’ve never heard of the term before today, it’s likely that you’ve worn or used a screen-printed product at some point without even realizing. The process is sometimes called serigraphy or silk screen printing, but all of these names refer to the same basic method.

This technique, in contrast to flexographic and gravure printing, is a method that inherently allows for the formation of a very thick wet layer and thereby also very thick dry films, which can for example be useful for printed electrodes where high conductivity is needed. The typical wet layer thicknesses are in the range of 10–500 microns. There are two types of screen printing: flat-bed screen printing and rotary screen printing. The principle of the two methods is the same. The squeegee moves relative to the screen and forces the ink paste through the opening of the mesh, which define the desired motif. There are significant differences in the operation of the two techniques. The advantages of flat-bed screen printing. For development and laboratory work this is a clear asset. In terms of production it is also possible to print on very large areas (on the scale of 10 square meters). Rotary screen printing differs in that the ink is contained inside the rotating cylinder with a fixed internal squeegee and the ink is less exposed to the surroundings. The mask is a lot more expensive than the flat-bed printing mask, but in terms of speed, edge definition/resolution, and achievable wet thickness, rotary screen printing is by far superior to flat-bed screen printing by at least an order of magnitude as it is a true roll-to-roll printing technique. It is the two-dimensional printing technique that allows for the largest wet thickness achievable (> 300 micron).

Because of the cost of the mask, the more delicate operation, the more difficult adjustment, and the relatively time-consuming cleaning procedures, rotary screen printing is not as well suited for laboratory work as the flat-bed technique. This technique has been shown to be effective in functional printing of conductive layers like printed film antenna or batteries applications. Currently, the back electrodes in large-scale produced OPV modules are mainly rotary screen printed on R2R machine 20, 21.

screen-printing

Inkjet-printing

Inkjet-printing is well known from home and office applications. During the recent decade, inkjet technology has made large inroads into the industrial domain. Several research studies at laboratory or industrial scale demonstrate the strong advance of digital and specifically of industrial inkjet printing.22

In fact, inkjet has become a mature technology for graphical applications. Even in functional printing like printed electronics, 3D, and bio/pharma/medical applications, there have been successful implementations of inkjet technology.

It works generating small droplets with high frequency to realize a pattern. Inkjet-printing head are made of one or several nozzle generating droplets. In particular, there have been several successful publications using inkjet printing to manufacture OPVs where the droplets are widely generated on demand (called drop-on-demand, DOD). The most recent ink-jet printing technology used piezoelectric crystal to form the droplets. Under an electrical field, a piezoelectric material is mechanically deformed. First, applying a negative voltage on the piezoelectric crystal allows to fill the nozzle decreasing its pressure. Then, a positive voltage leads to the droplet expulsion by increasing the pressure inside the nozzle. Inkjet-printing is a fully digital printing technique. The desired pattern is obtained using software: no mask or cylinder is required. More importantly,several studies have proven the great advantage of inkjet printing as a digital technology allowing freedom of forms and designs: large area OPVs with different artistic shapes were already demonstrated23.

Considering the fact that large area formation and roll-to-roll (R2R) processing can be done by inkjet printing, it can be a good choice for preparing homogeneous and thin layers for constructing OPV modules.

inkjet-printing

In these sections were presented the most widely used technics for solution-processed thin layer (particularly for OPV). Others printing and coating techniques could be used for the deposition of OPV layers. Interested readers should consult references presented at the end of this article.

Sheet-to-sheet (S2S) versus roll-to-roll

Doctor blading, spray-coating, slot-die coating, screen-printing, flexographic printing, gravure printing and inkjet-printing could be adapted for manufacturing line. Two main manufacturing ways could be distinguished for large scale production: sheet-to-sheet (S2S) and roll-to-roll (R2R).

For S2S process, the substrate, flexible or rigid, is a discrete sheet. S2S is suitable for the fabrication ofOPV modules, for instance for Internet of Things (IoTs) applications (lien vers la première newsletter : à verifier).

R2R is a well-established process used for instance for the printing of newspapers.  For this process, the flexible substrate is a continuous roll of material. It requires high amount of material and allows the production of large scale OPV modules, for instance for building integration. As a failure during the run could impact the whole production, discrete R2R could be preferred.

The following figure shows S2S (left) and R2R (right) industrial machines compatible with OPV production at large scale.

sheet-to-sheet
  1. Cui, Y. et al. Single-Junction Organic Photovoltaic Cells with Approaching 18% Efficiency. Adv. Mater. 1908205 (2020) doi:10.1002/adma.201908205.
  2. Li, S. et al. Asymmetric Electron Acceptors for High-Efficiency and Low-Energy-Loss Organic
  3. Hu, Z., Zhang, J., Xiong, S. & Zhao, Y. Performance of polymer solar cells fabricated by dip coating process. Sol. Energy Mater. Sol. Cells 99, 221–225 (2012).
  4. Sachse, C. et al. Transparent, dip-coated silver nanowire electrodes for small molecule organic solar cells. Org. Electron. physics, Mater. Appl. 14, 143–148 (2013).
  5. K. Norrman, A. Ghanbari-Siahkali, N. B. Larsen, Studies of spin-coated polymer films, Annu. Rep. Prog. Chem. Sect. C 101 (2005) 174–201.
  6. Frederik C. Krebs, Fabrication and processing of polymer solar cells: A review of printing and coating techniques, Solar Energy Materials & Solar Cells 93 (2009) 394–412.
  7. Cui, Y. et al., Single-Junction Organic Photovoltaic Cells with Approaching 18% Efficiency. Adv. Mater. 1908205 (2020) doi:10.1002/adma.201908205.- Lingxian Meng er al., Organic and solution-processed tandem solar cells with 17.3% efficiency. Science 361, 1094–1098 (2018).

 

  1. Morteza Eslamian, Ultrasonic Substrate Vibration-Assisted Drop Casting (SVADC) for the Fabrication of Photovoltaic Solar Cell Arrays and Thin-Film Devices, Nanoscale Research Letters (2015) 10:462.
  2. Yuanbao Lin et al., Printed Nonfullerene Organic Solar Cells with the Highest Efficiency of 9.5%, Adv. Energy Mater. 2018, 1701942.
  3. Guoqi Ji et al., 12.88% efficiency in doctor-blade coated organic solar cells through optimizing the surface morphology of a ZnO cathode buffer layer, J. Mater. Chem. A, 2019, 7, 212–220.
  4. L. Wengeler et al., Investigations on knife and slot die coating and processing of polymer nanoparticle films for hybrid polymer solar cells, Chemical Engineering and Processing 50 (2011) 478–482.
  5. Wan Jae Dong et al., Simple Bar-Coating Process for Fabrication of Flexible Top-Illuminated Polymer Solar Cells on Metallic Substrate, Adv. Mater. Technol. 2016, 1600128.
  6. Yuxiu Li et al., One-step synthesis of ultra-long silver nanowires of over 100 mm and their application in flexible transparent conductive films, RSC Adv., 2018, 8, 8057–8063.
  7. Andrea Reale Spraet al., y Coating for Poly.mer Solar Cells: An Up-to-Date Overview, Energy Technol. 2000, 00, 1 – 23.
  8. Yulia Galagan et al., Roll-to-Roll Slot–Die Coated Organic Photovoltaic (OPV) Modules with High Geometrical Fill Factors, Energy Technol. 2015, 3, 834 – 842.
  9. Jeongjoo Lee et al., Slot-Die and Roll-to-Roll Processed Single Junction Organic Photovoltaic Cells with the Highest Efficiency, Adv. Energy Mater. 2019, 1901805.
  10. J. M. Ding, A. F. Vornbrock, C. Ting, V. Subramanian, Pattern able polymer bulk heterojunction photovoltaic cells on plastic by rotogravure printing, Sol. Energy Mater. Sol. Cells 93 (2009) 459–464.
  11. Christos Kapnopoulos et al., Fully gravure printed organic photovoltaic modules: A straightforward process with a high potential for large scale production, Solar Energy Materials & Solar Cells 144 (2016) 724–731.
  12. Alem, S. et al. Flexographic printing of polycarbazole-based inverted solar cells. Org. Electron. physics, Mater. Appl. 52, 146–152 (2018).
  13. Frederik C. Krebs, Upscaling of polymer solar cell fabrication using full roll-to-roll processing, Nanoscale, 2010, 2, 873–886 | 873.
  14. Kim, J., Duraisamy, N., Lee, T.-M., Kim, I. & Choi, K.-H. Screen printed silver top electrode for efficient inverted organic solar cells. Mater. Res. Bull. 70, 412–415 (2015).
  15. Handbook of Industrial Inkjet Printing: A Full System Approach, First Edition. Edited by Werner Zapka – 2018 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2018 by Wiley-VCH Verlag GmbH & Co. KGaA.
  16. Eggenhuisen et al. High efficiency, fully inkjet printed organic solar cells with freedom of design. J. Mater. Chem. A 2015, 3, 7255–7272.

 

 

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Thin Film Processing Method: from lab to Industrialization of OPV Devices (1st part)

Thin Film Processing Method: from lab to Industrialization of OPV Devices (1st Part)

In comparison with the existing photovoltaic technologies, organic photovoltaic (OPV) is known as a low-cost technology. This is allowed by the fact that organic solar cells (OSCs) could be solution-processed. The main advantage of solution-processing is that OCs could be fabricated at ambient pressure and ambient temperature, reducing manufacturing costs. Over the last decade, the development of new efficient organic materials resulted in power conversion efficiencies (PCE) of organic solar cells (OSCs) exceeding 17% 1,2  at the laboratory scale but the majority of the produced devices have small dimensions and are manufactured by simple laboratory techniques like spin coating (this technique will be detailed in the following paragraphs) which generally wastes large amounts of materials and which allow to produce small device series.

To ensure large-scale production of these OPV devices, less wasteful methods compatible with continuous processes must be used. These can be broadly divided into coating techniques, which are suitable from the laboratory scale up to large scale manufacturing, and printing techniques, which are commonly adapted from large scale commercial processes and which have shown their effectiveness in several areas in particular for OPV.  In general, coating is used to describe a process by which a layer of ink is transferred to the substrate by essentially pouring, painting, spraying, casting or smearing it over the surface. The use of the word printing may also imply that a complex pattern is formed whereas coating generally does not infer this.

The aim of this newsletter is to introduce the reader to the main deposition techniques for the fabrication of OPV devices. As shown in our previous article, OSC is based on various layers. In a lab scale, some of these layers could be deposited by thermal evaporation under high vacuum. This technique allows very accurate control of the thickness of the deposited layer as well as low contamination of OSCs allowing to produce high quality of films. However, it is time-consuming and expensive as it is operated under vacuum. Thus, evaporation is not the most suitable deposition technique and solution processed OPV cells and modules are often preferred for industrialization. Based on this observation, next paragraphs focus on the main technics and their specific applications. First, solution-processed technics from lab to fab are presented, prior to describe how these technics could be adapted in a manufacturing line. The working principle of each printing will be illustrated schematically in the corresponding Figure. 

First part: Coating techniques widely used in laboratories

The solution processing presented in this section are widely used in laboratories. They allow to obtain quickly OSCs layers with simple ink formulation.

Dip Coating

During dip coating, the substrate is immersed in the coating solution. As it is withdrawn, a liquid layer is entrained on the substrate. The thickness of this entrained solution is determined by the withdrawal speed. A wet film with a well-defined thickness is dragged from the liquid upward along with the moving substrate. Despite its simple appearance, the dip-coating process involves a complex interplay between many counteracting factors: viscous drag upward on the liquid by the moving substrate, force of gravity on the wet film, surface tension in the concavely shaped meniscus, surface tension gradient along the height of the film due to drying effects, the disjoining (or conjoining) pressure. In general, dip coating requires a very fluid adhesive in the viscosity range of 50–500 mPas. This technique can be adapted to coat different surfaces (such as flat substrates or tubes). But it is very difficult to coat curved or flexible substrates. Several groups have demonstrated that the use dip coating technology as a fabrication tool allows to obtain efficient OSCs 3,4

dip-coating

Spin Coating

Spin-coating is the most widely used coating technique for OSCs layer deposition in laboratories scale during device optimization and materials screening. It involves the deposition of solution on a substrate and the spinning of this substrate at a chosen rotational speed. 5,6 The centrifugal force shears the solution, causing it to be distributed evenly across the surface as a thin film. The thickness of the deposited film is determined by the shearing force applied, which is proportional to the rate of rotation. It allows to obtain quickly homogeneous films with high reproducibility in thickness and morphology. This technique is perfect for use in research and development laboratories working on a wide range of thin-film technologies (such as photovoltaics and light-emitting diodes). However, it is a wasteful technic: most of the solution is ejected during spinning. Additionally, this can make it difficult to coat with a low-concentration solution. The main limitations of spin coating are that it is only effective for coating small substrates and is limited to batch processing. Spin coating is completely unsuitable for large-scale production of OPV devices, so this limits its applications to research and development.

It is important to mention that the highest lab efficiency and most of the world record in OPV were achieved using this technique for organic material deposition. 7

spin-coating

Drop Casting

Drop casting is a simple film forming technique used by many research groups as it does not require specific equipment. The solution containing desired material is cast on a substrate following by evaporation of the solvent. However, it is difficult to obtain uniform layer and to control its thickness (these parameters are fundamental for obtaining performing OPV devices)6,8. This technique is similar to spin coating, but the major difference is that no substrate spinning is required. Also, the film thickness and properties depend on the volume of the dispersion and concentration. Other variables which affect the film structure are substrate wetting, the rate of evaporation, and drying process. Volatile solvents are generally preferred for this technique which can wet the substrate. One of the advantages over spin coating is less material wastage.

Drop-casting

Doctor Blading

For Doctor-blading (also known as blade coating) which is a popular thin-film fabrication technique, a sharp blade is placed at a fixed distance from the substrate surface. The ink formulation is placed in front of the blade. Then the blade is moved linearly across the substrate leaving a wet film which is dried. The final thickness is a fraction of the gap between the substrate and the blade. The final thickness of the wet film will be influenced by the viscoelastic properties of the solution and the speed of coating. In comparison with spin-coating, doctor blading is more material-saving. Moreover, drying conditions are closer to those obtained with industrial processing, allowing to use it for the upscaling of OSCs layer deposition9,10. However, the wet layer film thickness has poor reproducibility. This is due to the shearing rate of the solution impacting the final film thickness. This technique is well suited for large-scale coatings and also well suited for creating thicker films from a viscous solution. It cannot offer the nanoscale uniformity or extreme thin films that spin coating can.

doctor-blading

Knife Coating

Knife coating11 as a R2R technique is similar to doctor-blading on a laboratory scale. The layer is deposited at a stationary knife, in front of which an ink reservoir is continuously supplying the meniscus standing between the web and the blade. The web movement ensures the layer deposition as it passes the knife. The wet thickness is related to the gap size between the knife and the substrate and to some extent also to the web speed. As a rule of thumb, the wet thickness is roughly half the gap size. Knife coating is suitable for deposition of homogeneous layers on large areas and can be carried out at high speed (>10 m/min).

knife-coating

Bar Coating

Bar coating (also known as Meyer bar coating) is very similar to doctor blading. An excess of solution is placed on the substrate and it is spread across by a bar. This bar is a spiral film applicator and is essentially a long cylindrical bar with wire spiraling around it. The gaps made between the wire and the substrate determine how much solution is allowed through. This subsequently determines film thickness. Coating speed, solution concentration and bar-substrate distance should be carefully adjusted in order to obtain optimized thin film13. With this technique, the thicknesses of films are limited to diameter of the wire. This technique is often used in the same fields as blade coating because of the similarities between the coating methods.

Flexible polymer solar cells were successfully demonstrated by optimizing the bar-coating process providing good uniformity of the deposited layers on reflective bottom electrode.12

Dip coating, drop casting, spin-coating, bar coating, knife coating and doctor blading are individual film-forming techniques processing of small substrate. If doctor blading could be compatible to manufacturing line, most of these technics do not allow patterning: the whole substrate is covered during processing. Thus, other technics are preferred for industrial production

Spray Coating

Spray-coating is based on sub-micrometric droplets ejection on a substrate. The aerosol is created by passing an ink through a nozzle. The path traveled by the nozzle over the wafer is optimized to ensure that the coating is applied evenly to the substrate. The solutions used in spray coating usually feature a very low viscosity, which guarantees that fine droplets form. Spray coating ensures a uniform layer even with high topography substrates, making it the preferred technique for structures of this kind. As techniques described previously, spray-coating coats the whole surface of the substrate. However, it is possible to use masks to pattern the layer. With this technique, the ink waste is not negligible. In particular, spray coating proved to be a valid technique for the realization of organic solar cells 14.

spray-coating

Slot die Coating

Slot-dies coating works pumping the ink through a coating head placed closed to the moving substrate. Solution flows through the head at a determined rate and the substrate is moved underneath it. It is a one-dimensional coating technique, allowing to depose strips of materials with a well-defined width. The exact amount of ink could be deposit, reducing material waste. One major advantage of this technique is that it is scalable. This means the process would work on an industrial scale. Additionally, it can be used in roll-to-roll processing. These factors make it ideal for use in manufacturing. However, this technique is a significantly more complex process, with multiple parameters needing be optimized. To create high-quality thin films, a deep understanding of the physics behind each variable is needed. This technique requires more initial training than the other techniques. It is a good investment if producing high-volume (when scaling up from development to manufacture) low-cost films but is often too expensive for initial research and development.

Due to its low solution wastage, wide range of coating viscosities, the high throughput speeds that can be achieved and the possibility for coating one side of rigid or flexible substrates, this technique is used in thin-film electronics especially in organic photovoltaics15, 16.

slot-die-coating

Although slot die coating is a powerful technique for fabricating OPV modules, its major drawback is that the patterning produced by the coating process is restricted to continuous stripes. In contrast, mechanical printing technologies such as gravure printing, flexography printing, screen printing and offset printing bring the advantage of large-area arbitrary shape and size using only additive process steps in sequence. This increases the freedom of product design and its integration into a range of applications

 

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